Recent progress of a laser diode (hereinafter noted as an LD) excited solid laser unit body wherein its output and luminance are highly developed makes it possible for a laser unit to conduct a precise welding processing or a fine removal processing with high speed and high precision that used to be impossible for a conventional process unit. Then the laser unit is used for spot welding or seam welding of an electric/electronic component or applied to a surface marking or scribing processing or a drilling or cutting processing of metal, semiconductor or ceramics.
As a representative example of a conventional solid laser unit, FIG. 9 shows an arrangement of a solid laser unit that mainly comprises an LD excited pulse type Nd:YAG laser unit body wherein a laser activated media is a rod-type Nd:YAG crystal and a mean output is in a 300 W class.
The Nd:YAG crystal 1 whose rod diameter is 5 mm and whose length is 116 mm is excited by LD light 3 radiated from an LD excited unit 2 equipped with 60 bars of LDs whose mean light output is 20 W/bar and that oscillates at a central wavelength of 808 nm, light of 1.06 μm irradiated from the Nd:YAG crystal 1 is selectively amplified between a total reflection mirror 5 and an output bond mirror 6 whose reflection coefficient is 70%, each of the mirrors 5, 6 constituting a laser resonator 4 whose resonator length is 400 mm, so as to be Nd:YAG laser light 7 and then the Nd:YAG laser light 7 is radiated from the output bond mirror 6. In addition, electricity is conducted to the LD excited unit 2 by a direct current stabilized power supply 8 and the Nd:YAG crystal 1 and the LD excited unit 2 are temperature-controlled directly or through purified water supplied by a purified water cooling system 9 to keep a temperature of its peripheral portion constant in order to maintain an Nd:YAG laser output stable.
In addition, a part of the Nd:YAG laser light 7 is made to be laser light 11 for monitoring by a beam splitter 10 and permeates through a power attenuator 12 and then is introduced into a high speed power sensor 13. Other laser light 7 is gathered by incident gathering optical system 14 so as to meet a transmission condition of an optical fiber 15 for transmission whose core diameter is 0.3 mm and whose length is 10 m. The laser light radiated from the optical fiber 15 for transmission is formed or focused by radiation gathering optical system 18 so as to be in a beam shape appropriate for processing an object to be processed 17 placed on a CNC table 16 and then a required laser processing is conducted.
In this arrangement, the laser light 7 is so made that current conducted to the LD excited unit 2 from the direct current stabilized power supply 8 is feedback-controlled so as to coincide a laser output value monitored by the high speed power sensor 13 with a command laser output value.
However, a conventional arrangement wherein a PIN type Si photo diode is used as a high speed power sensor of laser light for monitoring has following defects.    (1) Since an acceptable input light level to the power sensor is on an mW level, actual Nd:YAG laser light is required to attenuate to a degree of about one hundred thousandths by the use of a high-precision power split means in combination with a high attenuating means. However, a power split rate and a power attenuating rate of an optical component used as the high-precision power split means and the high attenuating means are easily changed in its characteristics due to humidity change or dust adhered to the optical component, which makes it difficult to predict actual Nd:YAG laser output accurately from the laser output for monitoring.    (2) Since temperature anaclisis of detecting sensitivity of the high speed power sensor is large such as 0.2˜1.0%/° C., the detecting sensitivity easily varies due to change of ambient temperature. Then it is difficult for the laser unit body placed under an environment wherein ambient temperature is not constant to predict actual Nd:YAG laser light output with accuracy in a stale manner from the measured laser output for monitoring.
As a result of this, the Nd:YAG laser unit wherein the laser output value for monitoring measured by the high speed power sensor is controlled as a feedback signal could not obtain stable Nd:YAG laser output with absolute precision of not greater than 2% due to the above-mentioned problems of the high power sensor. Then a problem was developed that processing defect such as accuracy defect or strength defect was inevitable for a laser processing using the laser unit body of the above-mentioned arrangement.
In addition, it is desired for the laser process unit using this kind of the pulse oscillating type solid laser unit to avoid processing defect due to fluctuation of laser output. The following is known as this kind of a laser process unit.
As a conventional example, FIG. 10 shows an arrangement of an LD excited pulse type Nd:YAG laser process unit body whose primary purpose is a laser welding processing wherein a laser activated media is a rod-type Nd:YAG crystal 101 and a mean output is in a 300 W class.
The Nd:YAG crystal 101 whose rod diameter is 5 mm and whose length is 116 mm is excited by LD light 103 radiated from an LD excited unit 102 equipped with 60 bars of 20 W/bar LDs that oscillates at a central wavelength of 808 nm, light of 1.06 μm irradiated from the Nd:YAG crystal 101 is selectively amplified between a total reflection mirror 105 and an output bond mirror 106 whose reflection coefficient is 70%, each of the mirrors 105, 106 constituting a laser resonator 104 whose resonator length is 400 mm, so as to be Nd:YAG laser light 107 and then the Nd:YAG laser light 107 is radiated from the output bond mirror 106. In addition, the Nd:YAG crystal 101 and the LD excited unit 102 are temperature-controlled directly or through purified water supplied by a purified water cooling system 108 to keep a temperature of its peripheral portion constant in order to maintain an Nd:YAG laser output stable.
A part of the Nd:YAG laser light 107 is reflected off a beam splitter 109, taken as monitor light 110 and introduced into a monitor light output measuring instrument 111 of a thermoelectric conversion type so as to measure output and other Nd:YAG laser light 107 permeating through the beam splitter 109 incomes into the incident gathering optical system 113 with the beam shutter open and gathers to an optical fiber 114 for transmission whose core diameter is 0.3 mm and whose length is 10 m so as to be transmitted. In case that the beam shutter 112 is closed the Nd:YAG laser light 107 incomes into a laser light output measuring instrument 115 of a thermoelectric conversion type so as to measure output.
Controlling and ON/OFF switching of the Nd:YAG laser output are conducted by controlling LD current by means of the direct current stabilized power supply 116 and generally LD current that is determined from a laser output characteristics obtained before the laser light is radiated outside is conducted corresponding to a desired laser output. In addition, monitoring the output of the Nd:YAG laser light 107 is conducted by comparing an output value of the monitor light 110 and a specified value of the monitor light 110.
The laser light radiated from an optical fiber 114 is formed or focused by radiation gathering optical system 119 so as to be in a beam shape appropriate for processing an object to be processed 118 placed on a CNC table 117 and then a desired laser processing is conducted.
Since a response speed is late such as about 0.1˜3 sec for the monitor light output measuring instrument 111 of a thermoelectric conversion type with which the conventional arrangement of the process unit is equipped, it takes time as generally the same as the response speed to detect an error even though the measurement can be done with high precision. Then in case that laser oscillating operation terminates in not exceeding the time constant or during a pulse oscillating operation wherein a pulse repetition frequency is not greater than 100 Hz, mean laser output can not be measured with sufficient accuracy as shown in FIG. 11. Then radiated laser output can not be monitored during a short pulse oscillating operating period or during a pulse oscillating operation of a low frequency that is not over the above-mentioned frequency although the output of the monitor light is measured.
As a result, when the laser output drops drastically due to damage of the total reflection mirror 105 and the output bond mirror 106 constituting the laser resonator 104 because of dust or dirt attached thereto, a problem is developed such as a processing is continued without a normal laser processing because detection of an error state such as output drop is delayed or the detection is impossible.